The present invention is in the field of metallized glassware and specifically relates to metallized glass-ceramic cookware exhibiting excellent range top cooking performance in combination with extended service life.
It is known to apply a thin metallic layer to the base of a glass vessel to reduce the incidence of thermal shock breakage which can occur, for example, when the heated vessel is placed on a cold surface. Hence, U.S. Pat. No. 2,053,923 to Stewart describes a glass vessel with a thin metallic bottom coating (e.g., a 0.003-inch thick coating of aluminum) which is of sufficient thickness to improve heat diffusion and dissipation as well as to add mechanical strength to the vessel.
It is also known to apply relatively thick metallic layers to glass-ceramic vessels to improve the heat distribution characteristics thereof in use, as shown, for example, by U.S. Pat. No. 3,523,013 to Small. Glass-ceramic materials are semicrystalline ceramic materials, which are produced by the controlled crystallization of glasses comprising so-called nucleating agents. Families of glass-ceramics are known which exhibit dense white opacity, high strength, and very low thermal expansion, so as to be eminently suitable for the manufacture of heat-resistant overware. Unfortunately, however, the thermal conductivity of glass-ceramic materials suitable for this use is low; therefore, even though they are not prone to thermal breakage, they are not readily adaptable to range top use because of very uneven temperature distribution when subjected to localized heating by a burner element.
As noted in U.S. Pat. No. 4,056,650 to Dates et al., substantial problems arise when it is attempted to join aluminum metal to low-expansion glass-ceramic cooking vessels, due to the very large difference in thermal expansion between these materials. Hence, metallic aluminum has a coefficient of thermal expansion of about 230.times.10.sup.-7 /.degree.C., while glass-ceramics of the kind useful for cooking applications have coefficients of thermal expansion below about 25.times.10.sup.-7 /.degree.C.,
The thermal stresses arising from this expansion mismatch over the range of temperatures encountered by cookware can theoretically approach 100,000 psi in the use of thick coatings (on the order of about 18 mils or more), a stress level which can cause cooking vessel failure in use. Yet a coating thickness of at least about 18 mils is necessary to impart adequate heat-distributing characteristics to an aluminum-coated glass-ceramic cooking vessel of conventional thickness.
In addition to the problem of thermal breakage attributable to stress applied by the aluminum coating, the service life of aluminum-coated glass-ceramic cooking vessels is limited by cracking and/or separation of the aluminum coating from the vessel. This behavior is also attributed to the large difference in thermal expansion and contraction between the vessel and the coating in use.
According to the teachings of the aforementioned Dates et al. patent, substantial improvements in the thermal durability and thermal shock resistance of aluminum-coated glass-ceramic cookware may be obtained by using a powder flame spraying process to apply the aluminum coating to the glass-ceramic vessel. In addition, the service life of the vessel may be extended by treating the aluminum coating with certain chemical agents prior to use.
Notwithstanding these improvements, the service life of aluminum-coated glass-ceramic cooking vessels is still not as great as would be desired. Moreover, the use of prior art chemical treatments to enhance vessel durability adds to the complexity of the manufacturing process and can increase the cost of a vessel to a level where it may not be competitive with other cookware designed for range top use.
It is therefore a principal object of the present invention to provide an economical aluminum-coated glass-ceramic cooking vessel exhibiting significantly improved service life. Other objects and advantages of the invention will become apparent from the following description thereof.